The Cambrian Period

Tectonic and climatic setting

See paleogeographic reconstruction in text

Spend a few minutes to locate the various continents and their names at this time.

North America = Laurentia

Northern Europe = Baltica

Gondwanaland = the southern continents.
They are upside down for the most part. You should be able to pick out the outline of Africa (far right),
Australia (upper left part of Gondwanaland), Antarctica (to the right of Australia).
A glance at the Ordovician map will show you South America upside down on the left, nestled on Africa.

As we proceed through the Paleozoic, we will be referring to these paleogeographic reconstructions frequently to
see changes as time passes.

For the Cambrian, note that the continents are separate (not all assembled into Pangea until the end of the Paleozoic),
and that they are primarily located in tropical latitudes.
Also note that there are NO CONTINENTS LOCATED AT THE POLES. This will become important later in the
Paleozoic because it influences climate.

What does this mean?
Basically, if there are no continents at the poles, there was no place for glaciers to grow.
Global temperatures were probably warm, because tropical waters could not flow far before running into a continent,
and being deflected N or S.

Also note the distribution of rock types, such as evaporites (E). Today, the desert latitudes are at about 30 degrees
N and S of the equator. Note all of the shallow sea deposits. Most are tropical, suggesting the sort of warm,
shallow sea conditions needed for the formation of limestones. Indeed, the Cambrian and Ordovician are times
of widespread deposition of carbonates, particularly in the Appalachian region.

Check the Cambrian Paleogeographic map to see the distribution of sedimentary environments.

Cambrian Fossil Record

The Cambrian is best described as a time of evolutionary explosion. All marine invertebrate phyla appeared during this time, with the exception of the bryozoa. The dominant marine invertebrates with hardparts were the trilobites, brachiopods, and archaeocyathids.
Tommotian (lowermost Cambrian)

Evidence:
modern low O2 environments have only small, soft-bodied forms.

Problems?
Muscles require collagen, suggesting that early muscles were weak, BUT
early evidence for metazoans shows burrows which would require good
muscles.

Stanley (1976) disagrees. Says Cambrian was a time of the initial proliferation of
organisms and life in general. Says skeletonization was a major evolutionary
experiment, which succeeded, leading to rapid adaptive radiation and survival of the
fittest.

The earliest organisms with soft bodies may have had respiration by diffusion across
body wall.
When O2 content of atm. increased, it allowed them to develop respiratory systems,
circulatory systems, etc. No longer needed to use O2 diffusion. Larger body size
possible.

A few worm-like animals and arthropods had specialized adaptations for feeding and may have
been carnivores.

One of the largest animals to live during the Cambrian was
Anomalocaris, about a half-meter long.
It had a pair of segmented, spiny appendages near its mouth. Its mouth was encircled by a ring of 32
radiating plates, resembling a slice of pineapple, with inward-facing spines, functioning like teeth in a
unique "jaw-like" apparatus. Its size and anatomy suggest that it was a predator. In addition, trilobites
seem to have "bites" taken out of them that appear to have been inflicted by Anomalocaris.
(Briggs, 1991, American Scientist).

The Cambrian was a time of "evolutionary experimentation" and adaptive radiation. Many
body plans were tested, but only a few succeeded during natural selection (predation,
competition, etc.).

Many groups of animals appeared, but few succeeded.

Trails: Climactichnites, 505 million years old, Late Cambrian, New York

The Burgess Shale

In a few places in the world, there are examples of exceptional preservation of soft-bodied marine invertebrates. The Burgess Shale is the most spectacular of these. Its occurrence early in the Cambrian serves to highlight the great diversity and variety of multicellular life at this early point in its history.

Wide variety of body plans is shown in the BURGESS SHALE. Although hard parts
had appeared, soft-bodied organisms were still present (as they are today).

Significance:
exceptional, unique preservation of soft-bodied animals in the Cambrian.

Discovered by Charles Walcott, 1909, Canadian Rockies, British Columbia.

Good preservation indicates deposition in anoxic conditions. Many delicate details of
soft part anatomy are preserved. (Legs and gills of trilobites, for ex.). Fossils found
in turbidite beds in the Burgess Shale, which were swept off an adjacent, well-
oxygenated, carbonate platform by turbidity currents, and killed and protected from
decay in anoxic water.